Pacemaker deriving its stimulation rate from blood pressure detected during diastole

Abstract: An apparatus (30) for determining a physical condition in a living being by determining peripheral resistance to flow is described. Peripheral resistance to flow is determined by measuring changes in the pressure drop characteristic in an artery during diastole. The pressure drop characteristic can be determined by measuring absolute pressure, relative pressure or dimensional changes in the artery during diastole. The apparatus (30) can advantageously be used in a rate-responsive heart stimulator (2).

Description: The present invention relates to a rate-responsive heart stimulator for sensing a physical condition in a living being, comprising a sensor for acquiring a measurement signal related to blood pressure in the living being. "Physical condition" refers to both the general state of health and the workload, i.e. whether the person is at rest or under slight or heavy physical strain. For people with various pathological heart conditions or functional defects in different tissues, the heart in particular, indications of the patient's state of health is desirable information. It is also desirable to have the ability to stimulate the function of the heart of patients with heart problems, so the function of a healthy heart is simulated as accurately as possible. Blood pressure is a parameter which reflects a person's state of health and which can also be utilized for controlling a heart stimulator's stimulation of a heart. In EP-A-0 178 528 is described a heart stimulator which can stimulate a heart at a variable rate according to systolic peak pressure or its time derivative. Here, a pressure sensor is placed in the right ventricle to measure blood pressure. Systolic pressure increases when the patient's level of physical activity increases, and an appropriate stimulation rate can be set according to the peak value or the time derivative of pressure. However, systolic pressure is a heart parameter which does not solely depend on the patient's degree of physical exertion but is also affected by other factors, such as stress, body position etc. In addition, there are natural variations in the systolic pressure of patients. So this all means that systolic pressure is not a particular appropriate parameter for use in attaining the most natural variation in stimulation rate possible. One object of the invention is to achieve a heart stimulator for sensing a physical condition related to blood pressure in a way which avoids the aforesaid problems. Another object of the invention is to achieve a rate-responsive heart stimulator which stimulates the heart at the most natural rate possible. The first object is achieved in accordance with the invention in that the heart stimulator comprises an analyzer for determining peripheral resistance to flow in the living being by determining a pressure drop characteristic for the drop in blood pressure in an artery during diastole on the basis of the measurement signal, the analyzer also comprising a timer for determining said pressure drop characteristic in the blood pressure. Peripheral resistance supplies a better measure of a person's physical condition, in particular a better measure of a person's degree of physical exertion, than blood pressure. Peripheral resistance affects the reduction or drop in arterial pressure during diastole. In principle, the drop in arterial pressure is exponential. Determination of the characteristic, or the time constant, for the pressure drop results in accurate determination of peripheral resistance. It is in connection with this an advantage if the analyzer coomprises a timer for measuring the duration for the drop in blood pressure from a first blood pressure level to a second blood pressure level. In this way, the time constant can be determined for the drop in pressure between two blood pressure levels. It is an advantage here if the first blood pressure is the blood pressure at the beginning of diastole and the second blood pressure is the blood pressure at the end of diastole. Alternately, the heart stimulator can be devised so the analyzer determines the drop in blood pressure over a preset time interval during diastole. Since the pressure drop characteristic is governed by the drop in pressure and the time required for the pressure to drop, the drop in pressure over a preset interval can reflect the pressure drop characteristic in the same way as a measurement of the time it takes pressure to drop from one blood pressure level to another. One refinement of the stimulator is achieved in accordance with an embodiment of the invention in that the sensor comprises a pressure-sensitive measurement body, devised for placement in an artery, for sensing the blood pressure. The pressure drop characteristic can be determined in any artery in the body. The most appropriate location for the measurement body should be decided from case to case. Some sites which may be preferable are e.g. the aorta, carotid artery or subclavian artery. It is, in connection with the above embodiment, advantageous if the pressure-sensitive body is devised to sense relative changes in pressure. In principle, the pressure drop characteristic is completely independent of absolute blood pressure and it is therefore fully sufficient only to determine relative changes in pressure in the blood circulation, which is simpler than measuring absolute pressure. An alternative embodiment of the heart stimulator is achieved in accordance with the invention in that the sensor comprises an impedance meter, devised for placement in or near an artery, preferably the aorta, in order to measures changes in the dimensions of the artery, said dimensional changes corresponding to changes in pressure in the artery. No measurement of pressure is actually necessary. As a result of the elastic properties of the arteries, the dimensions of the arteries change when pressure changes. Measuring these dimensional changes also supplies a measure of relative changes in pressure. Measurement of impedance is an effective method for determining these dimensional changes. Impedance can be measured inside the artery itself or from the outside of the artery. In conjunction with the figures, the invention will be described in more detail in the form of embodiments, whereby FIG. 1 shows a rate-responsive heart stimulator comprising an apparatus according to the invention; FIG. 2 shows in a block diagram the heart stimulator's construction in greater detail; FIG. 3 shows one possible version of the apparatus; FIG. 4 shows a second possible version of the apparatus; FIG. 5 shows a third possible version of the apparatus. A heart stimulator 2 is connected, as shown in FIG. 1, to a heart 4 in order to stimulate same. An atrial electrode 6 is anchored in the right atrium of the heart and connected, via a first electrode conductor 8, to the heart stimulator 2. A stimulation pulse can be generated by the heart stimulator 2 and delivered to the heart 4 via the first electrode conductor 8 and the atrial electrode 6. The stimulation pulse then returns to the heart stimulator 2 via body tissue and an indifferent electrode 10. A ventricular electrode is anchored in the right ventricle of the heart 4 and connected to the heart stimulator 2 via a second electrode conductor 14. In the corresponding manner as for the atrium, the heart stimulator 2 can stimulate the ventricle by emitting a stimulating pulse through the second electrode conductor 14 and the ventricular electrode 12. Even in this instance, the stimulation pulse returns to the heart stimulator 2 via body tissue and the indifferent electrode 10. For stimulation of the heart in a way which simulates the natural beat of a healthy heart, a pressure-sensitive measurement body 16 is placed in an artery outside the heart 4 to sense the pressure. In principle, the measurement body 16 can be placed in any artery. The measurement signal is sent to the heart stimulator 2 via a measurement signal line 18. On the basis of the measurement signal, the heart stimulator 2 can ascertain the patient's level of physical exertion and therefore regulate the stimulation rate in a way described below. The construction of the heart stimulator 2 is shown in greater detail in the block diagram in FIG. 2. An atrial stimulation pulse generator 20 is connected to the atrial electrode 6 and the indifferent electrode 10. The atrial stimulation pulse generator 20 generates and emits stimulation pulses with a programmable amplitude and duration. An atrial detector 22 is connected in parallel across the atrial stimulation pulse generator's 20 output terminal to sense spontaneous responses in the atrium. An atrial stimulation pulse can be inhibited if the atrial detector 22 senses a spontaneous atrial event. As a result no unnecessary stimulation pulses, which are uncomfortable to the patient, are emitted, and the heart stimulator's 2 power consumption is reduced. The atrial stimulation pulse generator 20 and the atrial detector 22 are controlled by a control device 24 which also receives signals from the atrial detector 22 when a spontaneous atrial event has occurred. The control device 24 regulates the stimulation pulse generator's 20 emission of stimulation pulses. In the corresponding manner, a ventricular stimulation pulse generator 26 is connected to the ventricular electrode 12 and the indifferent electrode 10. A ventricular detector 28 is connected in parallel across the ventricular stimulation pulse generator's 26 output terminal to sense spontaneous ventricular events. A ventricular stimulation pulse can be inhibited when a spontaneous ventricular event occurs. Thus, the heart stimulator 2 senses the heart 4 for spontaneous events, and no stimulation pulses are emitted as long as the heart works normally. The heart stimulator 2 is equipped with a workload detector 30 for determining whether the heart operates at an adequate rate even during physical exertion and for stimulating the heart 4 at an adequate rate during physical exertion. The workload detector 30 is connected to the pressure-sensitive measurement body 16 and processes the measurement signal obtained via the measurement signal line 18 in order to determine the heart stimulator bearer's level of exertion. Here, the control device 24 receives a signal from the detector 30 which can be transformed into a stimulation rate. The control device 24 can also control the workload detector 30 and/or supply it with other parameters. The heart stimulator 2 can, via a telemetry unit 32, transmit and receive information to/from a programming unit 34. Here, a physician can read out how the heart stimulator 2 has operated with the prevailing programming and the therapy supplied and determine how well the patient's heart 4 has operated spontaneously. The physician can also change the heart stimulator's 2 programming in order to improve the heart stimulator's 2 performance. In FIG.3 is shown a first example of the way in which the workload detector 30 can be devised in order to determine the bearer's level of activity. A measured pressure signal is sent from the measurement body 16, via the measurement signal line 18, to a signal conditioning unit 36 in the detector 30. The pressure signal from the signal conditioning unit 36 is compared in a pressure signal comparator 38 with two pressure levels. When the pressure signal drops below the first pressure level, a timer 40 is activated which measures the time it takes pressure to drop by a specific amount, governed by the two pressure levels. When the pressure signal falls below the second pressure level, therefore, the timer 40 stops, and the time measured is sent to the control device 24 which selects, by means of an algorithm or a table, a corresponding stimulation rate. The two pressure levels can be regulated by the control device 24. As an alternative to measurement between two preset pressure levels, the measurement body 16 can be devised so it only senses the relative change in pressure, and the pressure signal comparator 38 is devised to activate the timer 40 at peak pressure and stop the timer 40 at nadir pressure, the pressure drop during diastole constituting the pressure level for which the pressure drop time is determined. In FIG. 4 is shown another version of the workload detector 30 for determining the bearer's level of physical level. As in the preceding instance, a measurement signal is sent from the measurement body 16 via a measurement signal line 18 to a signal conditioning unit 42. The pressure signal is then sent to a pressure signal comparator 44 which activates a timer 46 when the pressure signal is less than a preset pressure level or exceeds its peak value. The timer 46 closes a switch 48 for a programmable period of time. When the switch 48 is closed, the pressure signal is sent to a pressure drop meter 50 which measures the drop in pressure during the preset interval. The pressure drop obtained is sent to the control device 24 for conversion to a stimulation rate. The control device 24 can change the timer's 46 timing interval. The measured drop in pressure over a preset interval supplies a pressure drop characteristic which is equivalent to the one obtained when the time for a specific pressure drop is measured. In FIG. 5 is shown an additional, alternative version of the workload detector 30. A signal is passed between a first electrode 52 and a second electrode 54 from a measurement signal generator 56. The signal is devised to measure impedance between the first electrode 52 and the second electrode 54 with an impedance meter 58. When the electrodes 52, 54 are placed so they measure dimensional changes in e.g. the aorta, the measured impedance is equivalent to changes in pressure in the aorta. The impedance signal obtained is sent to an analyzer 60 for additional signal conditioning. This process can be accomplished as described previously, i.e. the changes in impedance can be used for determining the pressure drop characteristic during diastole, and used for establishing the bearer's level of activity and determining a stimulation rate. The analyzer 60 communicates with the control device 24. The described heart stimulator can even be devised with bipolar electrodes instead of unipolar electrodes. In other words, the detector 30 can be incorporated in all kinds of known heart stimulators, including pacemakers, cardioverters and defibrillators, in order to sense the bearer's level of activity.

Claims: Rate responsive heart stimulator (2) comprising a stimulation pulse generator (20, 26) for generating and delivering stimulation pulses to a heart (4) at a variable stimulation rate, a detector (30) for sensing physical workload in a living being and a control device (24) for controlling the stimulation pulse generator's (20) stimulation rate on the basis of the workload sensed by the detector, and a sensor (16; 52, 54) for acquiring a measurement signal related to blood pressure in the living being, characterized by an analyzer (36, 38, 40; 42, 44, 46, 48, 50; 60) for determining the peripheral resistance to flow in the living being by determining a pressure drop characteristic for the drop in blood pressure in an artery during diastole on the basis of the measurement signal, said analyzer also comprising a timer (40) for determining said pressure drop characteristics in the blood pressure. Stimulator according to claim 1, characterized in that said timer is used for measuring the duration for the drop in blood pressure from a first blood pressure level to a second blood pressure level. Stimulator according to claim 2, characterized in that the first blood pressure level consists of the blood pressure at the beginning of diastole and the second blood pressure consists of the blood pressure at the end of diastole. Stimulator according to claim 1, characterized in that the analyzer (42, 44,, 46) determines the drop in blood pressure over a preset time interval during diastole. Stimulator according to any of the above claims, characterized in that the sensor (16) comprises a pressure-sensitive measurement body (16), devised for placement in an artery, for sensing blood pressure. Stimulator according to claim 5, characterized in that the pressure-sensitive measurement body (16) is devised to sense relative changes in pressure. Stimulator according to any of the claims 1-4, characterized in that the sensor (52, 54) comprises an impedance meter, devised for placement in or near an artery, preferably the aorta, in order to measures changes in the dimensions of the artery, said dimensional changes corresponding to changes in pressure in the artery.

Inventors: HEDBERG SVEN-ERIK, HEDBERG, SVEN-ERIK
Applicants: ST JUDE MEDICAL, ST. JUDE MEDICAL AB
Application year: 1994
Main classification: A61B5
IPC codes: A61B5, A61N1